Pulse width modulation is a technique that has been developed for AC and DC power supplies. The technique offers improvement over older techniques, such as linear voltage regulators, in size, efficiency and cost.
Pulse width modulation can advantageously be used in DC power supplies and AC inverters or frequency changers. In a common type of DC power supply, using pulse width modulation techniques, one or more semiconductor switches and an inductor are connected in series between a DC source and a DC load. The voltage across the DC load is dependent upon the voltage of the DC source and the relative on time or duty cycle of the semiconductor switches. The duty cycle of the semiconductor switch can be varied to vary the output voltage or to compensate for changes in the input voltage or load current. Typically, the circuit which controls the duty cycle of the semiconductor switch has inputs from the load voltage and from a reference voltage which are continuously compared to maintain the DC output voltage at a substantially constant level. Typically, the semiconductor switches are operated at a frequency of 20 kHz or more.
When an AC output is required, a pulse width modulated inverter is typically used. One simple but useful type of pulse width modulated inverter is called a half bridge. It approximates a sine wave output by switching the voltage source of an inductive element between two DC buses of opposite polarities. Semiconductor switches connecting the inductive element to the two buses are switched at a rate higher than the frequency of the desired output voltage. In a manner similar to a pulse width modulated DC supply, the duty cycle of the switching element determines the relative magnitude of the output voltage with respect to the two DC input buses. In a typical power supply known to the art, the duty cycle of the semiconductor switches vary in accordance with a sinusoidal or other AC reference voltage. Typically, the sinusoidal or other reference voltage is compared with a triangular wave signal having a fixed frequency higher than the frequency of the desired output voltage. The power elements (i.e., the semiconductor switches) are switched at the intersection of the two signals. If the comparison between the reference voltage and the triangular voltage also includes feedback from the output voltage, a closed loop system results which responds to changes in the output voltage caused by variations in the load current. Typically, carrier-modulated pulse width modulated inverters are not well suited for applying a load having large, rapid changes in the load current or in the desired output waveform.
Another type of pulse width modulated inverter has developed known as an optimum-response switching inverter. This type of inverter is illustrated in Electronics Engineers' Handbook, Second Edition, Section 15.33 (1982). As set forth in that reference, the switching rate of the inverter varies throughout the cycle and is determined by the amount of hysteresis which is included in a feedback path from the output voltage to the switching control circuit. Typically, the inverter requires very high switching rates in order to keep the error in the output voltage very small. These types of inverters have the disadvantage that the filter circuit which removes ripple at the switching frequency must be designed to operate over the variations in frequency throughout the AC cycle. Thus, a need exists for a pulse width modulated inverter which responds to rapid variations in the output loading or in the desired output waveform while holding the switching frequency substantially constant under steady state, or near steady state conditions.